With known back rests of the generic type the concealing shell is provided with arrowhead-like locking pins thus having barbed-like heads, while receiving sleeves which are correspondingly undercut are provided for the locking pins at the rear side of the base plate. The manufacture is relatively complicated, as the locking pins on the one hand and the receiving sleeves on the other hand cannot be manufactured of a cheap plastics material, but have to be glued, if for the base plate and the concealing shell themselves relatively simple plastics materials are used respectively. Furthermore, the assembly of the concealing shell to the base plate is in fact relatively simple, even if a high amount of force is necessary; a releasing, however, is very difficult to accomplish and most often results in breaking off at least one part of the locking pin, so that a renewed assembly cannot reliably be carried out.